I. Field of the Invention
The present invention relates to laminated heated glazing comprising at least two superposed transparent and mechanically strong substrate panes with interposition, between two adjacent panes, of an interlayer made of a transparent plastic, said glazing furthermore including, in its thickness or on the surface, at least one thin transparent conductive film that extends over at least part of the glazing, said film or films being heated for the purpose of deicing and/or demisting by the Joule effect, with a heating area between two current leads in the case of a single-phase current supply, or with three heating areas between their own current leads in the case of three-phase current supply, said current leads being placed on the boundary of the glazing and being connected to a current source external to the glazing, at least one thin film having flow separation lines formed by etching, in order to guide the current from one band to another.
II. Description of Related Art
In general, the deicing requires high electric power, of around 70 watts/dm2, whereas the demisting requires lower power, between 15 and 30 watts/dm2.
Examples of such laminated glazing that may be mentioned include vehicle windows, particularly aircraft cockpit windows, a typical (but not limiting) example of the structure and manufacture of which may be described as follows:
Structure:
The glass windows of an aircraft cockpit generally consist of three glass panes A, B, C starting from the exterior of the aircraft, these being joined together by thermoplastic interlayers, especially made of polyvinyl butyral (PVB) or thermoplastic polyurethane (TPU). In general, the internal face of pane A (face 2 when referring to FIG. 1, which will be described later) or the external face of pane B (face 3) is coated with the aforementioned thin heating film, this usually being based on a metal oxide such as SnO2 or ITO (tin-doped indium oxide). The film on face 2 is used for deicing while the film on face 3 is used for demisting the glazing. It should be pointed out that the demisting may also take on pane B as face 4, or on pane C, as face 5. Typically, the thickness of pane A is 3 mm while that of pane B is 5 or 6 mm in the case of an aircraft cockpit side window.
Formation of the Current Lead Bands:
A thin transparent conductive film heats by the Joule effect between current lead bands (electrodes or collectors or busbars) for example based on conductive enamels filled with silver (silver paste), these bands having been deposited on the internal face of pane A or of pane B by screenprinting. They generally have a width of 5 to 10 mm and are, in the case of a single-phase current supply, deposited over the entire length (or the width) of pane A or of pane B, generally a few millimeters from the edge of the window so as not to impede vision. The leads define between them the largest possible heating area relative to the total area of the glass panes, for the purpose of maximum deicing/demisting efficiency. As indicated, with a single-phase supply, there are only two leads. With a three-phase supply, the network comprises three areas with their own leads which may be connected in star connection or in delta connection.
When the window is flat, as very often in the case of windshields, pane A, after cutting and shaping, is semi-toughened in a vertical toughening furnace, the toughening thus breaking the silver enamels.
When the window is curved, pane B, after ???, to form a soft, rounded edge, and bending, it is chemically strengthened so as to increase its modulus of rupture. As an example, the pane made of a glass having a composition that allows chemical strengthening down to a great exchange depth, of greater than 250 microns, (for example, the glass Solidion®) is used. The current leads are screen printed on the glass and then baked at high temperature after the chemical strengthening step.
Formation of a Thin Heating Film:
The next step consists in depositing a thin transparent conductive film on the face of the glass pane provided with its current leads.
In the case of an ITO film in particular, the deposition is advantageously carried out by a PVD (Physical Vapor Deposition) technique which consists in sputtering source material by the ions extracted from a plasma. If the source material, called target, is negatively biased to initiate the plasma, the technique is cathode sputtering. If an electric field is added to the perpendicular magnetic field, so as to increase the ion density in an area close to the cathode, this is called magnetron sputtering. Mention may be made of cathode sputtering with a ceramic ITO target on a “planar magnetron”, which deposits ITO over the entire surface of the glass. The glass run speed and magnetron power parameters determine the resistance per square, and therefore the electrical resistance, between ???, relative to the voltage/current supply characteristics of the heated glazing.
Mention may also be made of the pyrolysis thin-film deposition process, in which technique the mixture composed of an organic part and a mineral part is sprayed, using a spray system, onto a glass heated to between 500 and 700° C., especially between 600 and 650° C., the organic part being burnt off and the mineral part remaining on the glass in the form of a thin film. With this technique, an SnO2-based film may in particular be deposited.
It is also possible to deposit the thin films by vacuum evaporation using a Joule effect.
In the most frequent case of nonrectangular glazing, the leads follow the geometry of the glazing and are therefore no longer parallel. The heating network must, in this case, have areas of different local resistance so as to compensate for the variation in distance between means. The filing company has means for calculating and producing intrinsically non-homogeneous networks so as to produce networks of any shape and of homogeneous dissipation, that is to say without cold spots in the sharp corners of the glazing assemblies. This results in particular in the production of curved lines or inflections for the flow separation so as to guide the current, these lines being formed by etching the thin heating film.
In the case of a three-phase supply, thin heating film is also etched so as to separate the heating network into three areas by etching phase separation lines.
Assembly/Autoclaving/Finishing:
To complete the manufacture of the window, pane A, provided with its thin heating film, together with a connection system based on a metal braid bonded to the current leads, is assembled with the other two glass panes, B and C, by means of thermoplastic interlayers, such as PVB or TPU interlayers. The assembly is baked in a vacuum bag within an autoclave at high pressure and high temperature, to give a complete laminated product. The edges of the laminate are sealed by a peripheral encapsulation by means of barrier materials of the polysulfide, ZEE stainless steel and overmolded silicone seal type, so as to allow the window to be fitted into the “aircraft” structure.
When a thin heating film is provided on pane B, such a film is applied before panes A, B and C are joined together.
The problem that arises is that the flow separation lines, which are formed with widths of 0.5 mm by the standard techniques in thin heating films, are visible through the glazing, thereby reducing the visual comfort and aesthetics of the glazing.